"It's truly a dark day for the Malaysia judiciary which has shown itself incapable of standing up straight when national political issues are in play in cases before them," Phil Robertson, deputy Asia head of group Human Rights Watch, said after the verdict.

Correspondents say Mr Anwar is seen as the key challenger to the ruling party, which has been in power since Malaysia's independence in 1957.

The Barisan Nasional coalition won 133 of the 222 seats in parliament in the 2013 elections - although it won the elections, it was the coalition's worst ever result.

Thousands of Malaysian opposition supporters rallied against the result, alleging that the polls were fraudulent.

Mr Anwar was previously a member of the Barisan Nasional coalition, but fell out with top leaders and was sacked in 1998.

He was then charged with sodomy and corruption, and given a six-year jail term for abuse of power, which sparked huge street protests.

In 2000 he was also found guilty of sodomy with his wife's driver and jailed for a further nine years.

In late 2004 Malaysia's Supreme Court overturned the sodomy conviction, freeing him from jail.

He then emerged as a leader in the opposition movement, leading it to increasingly strong performances in both the 2008 and 2013 elections.